It is a Motorola CP100 two-way radio. The CP100 is a 2W UHF or VHF radio, depending on the actual model. Its speaking distance would be about 3-4 miles in open terrain. However, the obstacles found on the Island(s) (Hydra concrete building, jungle, hills etc.) will decrease the talking distance significantly. However, this factor may change as there might be a repeater the Others use to overcome this problem.
It is not possible to find out the model, therefore it's not known whether it's a VHF or UHF radio.
People using Other's walkies
Jack, while holding Ben hostage during surgery, takes Tom's walkie and uses it to speak with Kate and Pickett.("I Do")